1. Field of the Invention
The present invention relates to an optical option attachment ring, and particularly to an optical option attachment ring that will not disengage until and after a fastening screw is tightened.
2. Description of the Related Art
There has been conventionally known an optical option attachment ring for mounting a wide-converter, a tele-converter or the like to a lens barrel (see Japanese Patent Application Laid-Open No. 2004-271892, for example).
The optical option attachment ring described in Japanese Patent Application Laid-Open No. 2004-271892 will be explained with reference to the drawings. As shown in FIG. 9, the optical option attachment ring 10′ includes one end 21′ and the other end 22′ disposed with a predetermined gap therebetween, a ring body 20′ that connects the one end 21′ to the other end 22′ and receives a lens barrel (not shown), and a fastening screw 23′ that is inserted into screw holes 21a′ and 22a′ that pass through the one end 21′ and the other end 22′. The optical option attachment ring is configured such that tightening the fastening screw 23′ brings the one end 21′ and the other end 22′ closer to each other and hence the diameter of the ring body 20′ decreases so that the optical option attachment ring 10′ is fastened to the lens barrel, while loosing fastening screw 23′ separates the one end 21′ and the other end 22′ from each other and hence the diameter of the ring body 20′ increases so that the optical option attachment ring 10′ disengages from the lens barrel.
As shown in FIG. 9, the one end 21′ and the other end 22′ of the optical option attachment ring 10′ have inclined portions, and tightening the fastening screw 23′ moves a part 22b′ of the other end 22′ along the inclined surface of the one end 21′, so that the part 22b′ projects inward from the inner circumferential surface 20a′ of the ring body 20′ and is situated (engaged) in a groove (now shown) formed in the lens barrel. This prevents the optical option attachment ring 10′ from disengaging from the lens barrel even if the fastening screw 23′ loosens.